1. Field of the Invention
This invention relates to non-impact printing and more particularly to a method and apparatus for non-impact printing using printheads with plural recording elements wherein correction is required to provide uniformity in recording by the recording elements.
2. Description of the Prior Art
In the prior art of non-impact printing such as with use of LED printheads, examples of which may be found in U.S. Pat. Nos. 5,255,013 and 5,253,934, it is known that correction of the recording elements; i.e., LEDs, is often required due to non-uniformity in light output of these elements. Typically, a non-uniformity correction look-up table (LUT) is provided to adjust exposure times so that at any one required grey level all the light-emitting diodes (LEDs) can be enabled to output a uniform amount of exposure energy. This can be achieved by adjusting exposure times and/or intensities so that weaker emitters are enabled, say, for longer exposure times than stronger emitters so that the exposure energy from each emitter is uniform.
Typically, correction data is determined through factory measurement of the brightness of the individual recording elements while the printhead is off of the copier/printer apparatus. The correction data may then be calculated and loaded into the copier/printer apparatus. When the printhead is then mounted onto the copier/printer, correction tables associated with the copier/printer may be used to correct the image data.
For each level of gray the number of exposure times that can be requested is potentially equal to the number of LEDs in the printhead. Thus, for a j-bit gray level system that uses an LED printhead with N LEDs as the writer, the number of possible exposures that can be requested is N (2.sup.j -1), excluding the null exposure level (white). For N=4000 (typical) and j=4, this number is quite large. There is no economical printhead architecture that is capable of generating nearly as many exposures as this. The number of exposures that a typical LED printhead can generate is determined by its controller circuit. For a printhead with a k-bit (say 6-bits) controller, the total number of exposures that can be generated is 2.sup.k, with j.ltoreq.k but significantly less than N (2.sup.j -1). It is the goal of a non-uniformity correction algorithm such as that disclosed in U.S. application Ser. Nos. 08/175,079 and now U.S. Pat. No. 5,666,150 and 08/580,263 and now U.S. Pat. No. 5,818,501, the contents of which are incorporated herein by reference, to use the total number of exposures that can be requested to generate an optimum look-up table (LUT) with 2.sup.k entries. The procedure for generating the LUT must condense in some optimal manner all the exposures that can be requested (including zero) into 2.sup.k exposures.
I have used a frequency analysis technique to identify the spatial exposure non-uniformity noise of a printer system before and after non-uniformity correction. Because known non-uniformity correction methods are used to correct on a pixel-by-pixel basis for binary and multilevel printheads, a typical exposure error (in %) curve vs. spatial frequency would show a curve that has fluctuation in residue error more or less uniformly distributed in frequency space as shown in FIG. 9. FIG. 9 is a plot of the exposure error (in %) for gradation level 12 using a k=6 bits LED printhead (600 dpi) after the known non-uniformity correction method of U.S. application Ser. No. 08/175,079 has been applied with a dual LUT (a 256 levels bin LUT for the LED pixel brightness and a 6-bits output correction LUT that uses the bin output as input to select the proper 6-bits exposure time output) approach described in U.S. application Ser. No. 08/580,263
It is known that human observers have a higher tolerance to non-uniformity noise at higher spatial frequency than lower spatial frequency both for continuous tone, binary halftones and multilevel halftones. To further improve on the non-uniformity correction of multiple pixel printing system such as an LED printhead (or inkjet or thermal printhead), it would be desirable to be able to push the spatial frequency of the residue non-uniformity exposure error to a spatial frequency less objectionable to a human observer.
If non-uniformity correction is handled solely on a pixel-by-pixel basis, the residue non-uniformity in exposure is not correlated to neighborhood pixels. Thus, if a block of pixels start out with a similar brightness but each individual pixel can only be correlated to .about.+/-2.5% for a particular gradation level (for example one out of the 16 gradation levels available for a k=6 bits printhead), then one can obtain a corrected printhead wherein a block of LEDs will all be giving 2% more exposure than the average and another neighborhood block of LEDs may all be corrected to 2% less exposure than average. Defects like this are very visible to a human observer. As will be appreciated from the above, while each individual LED has been provided with excellent correction, there still may arise visual printing artifacts that detract from the print and may ironically result from the correction process.
It would, therefore, be desirable to provide for a method and apparatus that overcomes these problems.